Photo? Please be more specific.
Is it a silver print?
Fibre or RC?
Is the image stuck to the glass because the gelatine (fibre print) has got damp?
Or is there some substance in there doing the sticking?

If it is a fibre silver print and the gelatine has stuck then just soak the print and glass in warmish (25C) water for a few hours and try carefully peeling with the assistance of a flat edge like a spatula.
If is does not seem to be peeling easily then soak for a bit longer and try again.
Do not soak for too long or the gelatine might soften too much and start to part company with the base. For the same reason do not have the water over 25C.
If it comes off then rinse the print and dry under pressure to keep it flat.
You might lose some specks of print surface but you can retouch.

RC paper will sometimes unstick with the same basic method but you have to soak for a bit, loosen the edge a little then soak some more, loosen a little bit more. And so on.

The whole process is like getting a label off a bottle or jar.
If it doesn't come off easily don't force it.

If it is stuck with some substance then you need to find the right solvent.
If it isn't a silver print then you have the problem of possibly dissolving off the image.
Colour photos will succumb to the same techniques as silver prints - they are usually RC though.

In extreme cases (I used to do some conservation work for some Museums in my spare time) you have to carefully remove the glass itself a bit at a time. Not impossible but time consuming and very delicate work.

It'll be RC then. Must have got some damp in somehow. Probably boiling the sprouts for that month before Christmas.
Soak in lukewarm water for about 5 minutes. A little wetting agent might help.
Tease around the edges.
Soak a bit longer.
Tease the edges...
And so on. It can be a long, slow, laborious process.
You could try putting it to soak and slowly adding hot water to warm the whole thing up gently. Get it up to 50 or 60 C and then take it out.
Slowly heating the glass makes it expand without cracking and as the print won't it might unstick it. But there is a risk of only unsticking part and the expansion might fracture the emulsion.
It's like untangling a ball of string. It takes patience and perseverance.

For future reference, I usually window mount prints in clip frames using paper to make the window. Keeps the glass a fraction of a mil away from the print and avoids such problems. This is why you always card window mount in 'proper' frames.